The most commonly employed photographic elements are those which contain a radiation sensitive silver halide emulsion layer coated on a support. Although other ingredients can be present, the essential components of the emulsion layer are radiation sensitive silver halide microcrystals, commonly referred to as grains, which form the discrete phase of the photographic emulsion, and a vehicle, which forms the continuous phase of the photographic emulsion.
It is important to recognize that the vehicle encompasses both the peptizer and the binder employed in the preparation of the emulsion layer. The peptizer is introduced during the precipitation of the grains to avoid their coalescence or flocculation. Peptizer concentrations of from 0.2 to 10 percent, by weight, based on the total weight of emulsion as prepared by precipitation, can be employed.
It is common practice to maintain the concentration of the peptizer in the emulsion as initially prepared below about 6 percent, based on total emulsion weight, and to adjust the emulsion vehicle concentration upwardly for optimum coating characteristics by delayed binder additions. For example, the emulsion as initially prepared commonly contains from about 5 to 50 grams of peptizer per mole of silver, more typically from about 10 to 30 grams of peptizer per mole of silver. Binder can be added prior to coating to bring the total vehicle concentration up to 1000 grams per mole of silver. The concentration of the vehicle in the emulsion layer is preferably above 50 grams per mole of silver. In a completed silver halide photographic element the vehicle preferably forms about 30 to 70 percent by weight of the emulsion layer. Thus, the major portion of the vehicle in the emulsion layer is typically not derived from the peptizer, but from the binder that is later introduced.
While a variety of hydrophilic colloids are known to be useful peptizers, preferred peptizers are gelatin--e.g., alkali-treated gelatin (cattle bone or hide gelatin) or acid-treated gelatin (pigskin gelatin)--and gelatin derivatives--e.g., acetylated gelatin or phthalated gelatin. Gelatin and gelatin derivative peptizers are hereinafter collectively referred to as "gelatino-peptizers".
Materials useful as peptizers, particularly gelatin and gelatin derivatives, are also commonly employed as binders in preparing an emulsion for coating. However, many materials are useful as vehicles, including materials referred to as vehicle extenders, such as latices and other hydrophobic materials, which are inefficient peptizers. A listing of known vehicles is provided by Research Disclosure, Vol. 176, December 1978, Item 17643, Section IX, Vehicles and vehicle extenders. Research Disclosure is published by Kenneth Mason Publications, Ltd., Emsworth, Hampshire P010 7DD, England.
It has been recognized that when the gelatin incorporated in an emulsion layer of a photographic element is oxidized, modification of emulsion photographic properties can result. Corben U.S. Pat. No. 2,890,215 discloses the desensitization of gelatin by treatment with a peracid. Komatsu et al Japanese Kokai No. 58(1983)-70221 discloses improved keeping stability for internal latent image forming silver halide emulsions when oxidized gelatin is employed. Komatsu et al Japanese Kokai No. 59(1984)-195232 discloses improved storage stability for silver halide emulsions having silver chloride grain surfaces prepared using oxidized gelatin.
Moll, "Investigations of Oxidized Gelatins", 2nd Photographic Gelatin Symposium, sponsored by the Royal Photographic Society, Oxford, United Kingdom, Sept. 6, 1985, discloses that the chemical and physical properties of oxidized gelatins, including luminescence of emulsions prepared therefrom, do not differ substantially from those of the native gelatin. The sensitometry and growth restraining properties, however, are reportedly changed by the oxidation treatment. It is stated that these changes cannot be attributed to oxidation of methionine.
Mifune et al EPO No. 0,144,990 A2 discloses a process for controlled ripening of a silver halide emulsion with a sulfur containing silver halide solvent. An oxidizing agent is relied upon to terminate ripening of the emulsion once the desired extent of ripening is accomplished.
Chloride, bromide, and iodide are the halides from which silver halide grains are formed. The highest photographic speeds are realized with silver bromide grains, optionally containing a minor proportion of iodide. The incorporation of chloride in silver halide grains is recognized to be advantageous for a variety of photographic applications. For example, silver chloride exhibits less native absorption in the blue portion of the visible spectrum than the remaining silver halides and can therefore be used with green or red spectral sensitizing dyes to record green or red light more selectively. Further, silver chloride is more soluble than the other photographically useful silver halides, thereby permitting development and fixing to be achieved in shorter times. Radiation sensitive photographic emulsions having halide grains containing chlorlde as the sole halide or in combination with bromide and/or iodide are the preferred emulsions for producing photographic prints.
Recently the photographic art has turned its attention to high aspect ratio tabular grain emulsions, herein defined as those in which tabular grains having an aspect ratio greater than 8:1 account for greater than 50 percent of the total grain projected area. These emulsions can offer a wide variety of advantages, including reduced silver coverages, thinner emulsion layers, increased image sharpness, more rapid developability and fixing, higher blue and minus blue speed separations, higher covering power, improved speed-granularity relationships, reduced crossover, less reduction of covering power with full forehardening, as well as advantages in image transfer. Research Disclosure, Vol. 225, January 1983, Item 22534, is considered representative of these teachings.
In almost every instance the advantages of high aspect ratio tabular grain emulsions are enhanced by limiting the thickness of the tabular grains. High aspect ratio tabular grain silver bromide emulsions having tabular grain thicknesses well below 0.3 .mu.m have been formed, and corresponding silver bromoiodide emulsions have been recently produced. High aspect ratio tabular grain emulsions the tabular grains of which are formed by chloride as the sole halide or in combination with bromide and/or iodide have been achieved with difficulty only by observing specific preparation requirements.
Wey U.S. Pat. No. 4,399,215 discloses the double jet precipitation of high aspect ratio tabular grain silver chloride emulsions. The process of preparation does not permit the initial presence of bromide or iodide ions and requires the presence of ammonia, a pAg in the range of from 6.5 to 10, and a pH in the range of from 8 to 10. While tabular grains are formed, the ripening action of the ammonia present during precipitation thickens the tabular grains. Thus, high aspect ratio tabular grain silver chloride emulsions prepared as taught by Wey are substantially greater than 0.35 .mu.m in tabular grain thickness.
Maskasky U.S. Pat. No. 4,400,463 discloses a process of preparing high aspect ratio tabular grain emulsions, the halide content of which is at least 50 mole percent chloride, based on silver. The process disclosed requires the use of aminoazaindene as a growth modifier and a synthetic peptizer. The peptizers disclosed to be useful are water soluble linear copolymers comprising (1) recurring units in the linear polymer chain of amides or esters of maleic, acrylic, or methacrylic acids in which respective amine or alcohol condensation residues in the respective amides and esters contain an organic group having at least one sulfide sulfur atom linking two alkyl carbon atoms and (2) units of at least one other ethylenically unsaturated monomer. Otherwise comparable emulsions prepared with no peptizer or with only gelatin as a peptizer did not produce a tabular grain emulsion.
Wey et al U.S. Pat. No. 4,414,306 discloses a a process for preparing high aspect ratio tabular grain silver chlorobromide emulsions the chloride content of which can range as high as 40 mole percent, based on silver. This is achieved by maintaining a molar ratio of chloride to bromide ions in the reaction vessel of from 1.6:1 to 258:1 and maintaining the total concentration of halide ions in the reaction vessel in the range of from 0.10 to 0.90 normal.
Collectively these patents teach that high aspect ratio tabular grain emulsions containing chloride as the sole halide or in combination with other halides can be achieved by accepting one or a combination of (1) tabular grain thicknesses greater than 0.35 .mu.m, (2) a synthetic peptizer other than gelatin, and (3) limiting the chloride to less than 40 mole percent of the total halide, based on silver.